REGULATOR Handle positive returncode from enable

This makes _regulator_enable() properly handle the case where
a regulator is already on when you try to enable it. Currently
it will erroneously handle positive return values as an error.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>